The 4 string tenor guitar was conceived as a doubling instrument for dance band tenor banjo players, so CGDA tuning was the norm. The same pitch as viola/mandola. "Chicago tuning" mentioned above is another option for guitarists, also sometimes used on 4 string plectrum guitar.( Think guitar with plectrum banjo neck) Or, indeed, any open tuning. Players of Irish dance music tend to favour GDAE one octave below violin/mandolin, using heavier gauge strings. Some modern makers are building instruments specifically for this lower tuning.Also there is the tuning used by Eddie Freeman on his "Eddie Freeman Special" guitar. CGDA again but with the D and A strings an octave lower, giving a dense close harmony for powerful chordal rhythm playing.